1. Field of the Invention
The present invention relates to dispensing a liquid. In particular, the present invention relates to dispensing slurry in a chemical-mechanical polishing (CMP) application.
2. Discussion of the Related Art
In sub-micron integrated circuits, CMP techniques are used to create the planarity required in multi-level interconnect structures. Specifically, to create a planar surface for depositing an interconnect layer, e.g. aluminum, tungsten, or copper, an interlayer dielectric (e.g., silicon dioxide) is planarized by a polishing process which uses an alkaline or acidic slurry with fine abrasive. One example of such a slurry includes fine silicon dioxide particles (e.g., average diameter of 70 nm) suspended in deionized water having an adjusted pH of approximately 11. The alkalinity can be provided by potassium hydroxide (KOH) and ammonium hydroxide (NH.sub.3 OH) For other applications, acidic slurries are used.
Typically, in a CMP polisher, the semiconductor wafer to be polished is held under slight pressure by a polishing head against a polishing pad. For a CMP application, the polishing pad is typically made of a polyurethrane material. During the polishing process, the polishing head moves the semiconductor wafer according to a predetermined motion pattern. Alternatively, the polishing pad can be mounted on a platform which moves relative to the semiconductor wafer. The motion of the semiconductor wafer relative to the polishing pad and the mechanical and chemical actions of the slurry, together provide a polishing action for planarizing the surface of the wafer. In such a process, because the slurry has a high content of water, the slurry is not only an abrasive, but also acts as a coolant which maintains the surface of the wafer to an acceptable temperature.
In the prior art, the slurry is typically sprayed onto the surface of the polishing pad by appropriately positioned nozzles linked by a slurry supply line, and delivered to the surface of the polishing pad at 50 to 500 cc per minute using a peristalic pump. However, a number of problems can develop over time under this arrangement. In particular, deposits from the slurry can clog the nozzles, leading to uneven and inadequate delivery. In some situations, uneven and inadequate slurry delivery can result in poor polishing uniformity and even severe damage to the wafer due to excessive heat. In addition, even when nozzles are in good working condition, it is difficult to control the amount of slurry that is dispensed. Further, it is important to provide a means of cleaning the slurry dispensing apparatus. When allowed to dry out, slurry forms flakes which, if deposited on semiconductor wafers, can degrade integrated circuit performance.